JAX-WS is the core Java web service technology for Java EE applications.
It provides a unified client and server-side API for writing SOAP/WSDL based web services.
JAX-WS is platform independent. Many Java platforms like Glassfish, Axis2 or CXF support JAX-WS.
Thus services developed with JAX-WS on one platform can be easily ported to another platform.

JAX-WS is based on annotations like @WebService thus greatly simplifying the development of web services.
POJOs (Plain Old Java Objects) can be simply annotated with JAX-WS annotations thus turning these into web service implementations.

JAX-WS is the core web service technology according to JSR-224 affording basic web service functionality.
WSIT (Web Service Interoperability Technology) sits on top of JAX-WS and adds enhanced functionality like security, reliability and transactions.
WSIT is the standard Java WS protocol stack beyond basic WS functionality (SOAP, WSDL) to achieve interoperability between Java and .Net (for more complex applications that go beyond simple WS requests).